The coatings industry is under continuing regulatory pressure to develop formulations that reduce the levels of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) contained in paints and other coatings. Traditional coating formulations, including polyurethanes, are solvent-based and often cannot meet the newer restrictions on solvent use. Coating formulations in which water partially or completely replaces organic solvents continue to evolve.
Initially, aqueous polyurethane dispersions (PUDs), which are one-component coating systems, appeared in response to higher solvent prices and the increased demand for low-VOC coatings. These are usually made by reacting mixtures of polyols and dimethylolpropionic acid with a polyisocyanate to give a complete polyurethane or an isocyanate-terminated prepolymer. This product is then dispersed in water (which may contain other isocyanate-reactive compounds) by neutralizing the acid groups with a base, typically a tertiary amine. While aqueous PUDs provide a low-VOC alternative to traditional two-component, solvent-based coating formulations, they have some disadvantages. Because they are only lightly crosslinked, coatings from aqueous PUDs often lack adequate solvent resistance, water resistance, gloss, hardness, and weathering properties. In addition, a cosolvent is usually needed for good coalescence, so solvents are not easy to eliminate from the formulations. Preferably, low-VOC coatings could be made without sacrificing important physical properties.
In the early 1990s, two-component aqueous polyurethane coatings arrived on the scene (see generally: P. Jacobs et al., "Two-Component Waterborne Polyurethane Coatings: Now and Into the Next Century" and cited references). Bayer scientists discovered that it is possible to use water as a carrier for reactive 2K systems and still get coatings with good appearance and physical properties. Two-component aqueous polyurethane coating formulations are dispersions of separate polyol and polyisocyanate moieties. A coating film forms after water evaporates and the components react to give a crosslinked polymer network. While 2K aqueous polyurethane coatings should, in theory, match the properties available from solvent-based 2K systems, the coatings have, in practice, lacked adequate water resistance, gloss, weatherability, and hardness.
The success of aqueous 2K systems has, until now, relied on some important and often unwieldy formulation twists. For example, the polyol required, which needs both hydroxyl functionality for the polyurethane-forming reaction and acid groups for water dispersibility, is usually not commercially available. In one approach (illustrated by U.S. Pat. No. 5,075,370), an acrylate polymer with acid and hydroxyl functionalities is made by copolymerizing (in a free-radical polymerization) an acrylic acid monomer and a hydroxyalkyl acrylate monomer (e.g., hydroxyethyl acrylate or hydroxyethyl methacrylate). Unfortunately, hydroxyalkyl acrylates are rather expensive. In addition, it is difficult to make hydroxyalkyl acrylate polymers that have both high hydroxyl functionality and molecular weights low enough to have value for low-VOC, crosslinkable coating systems. The result is a lower level of coating physical properties than would otherwise be desirable. Recently developed hydroxy-functional acrylate polymers based on allylic alcohols and alkoxylated allylic alcohols (see, e.g., U.S. Pat. No. 5,525,693) overcome some of the limitations of using hydroxyalkyl acrylate monomers. However, the value of these resins has, until now, been demonstrated primarily for solvent-based polyurethane coatings (see Examples 9-11 of the '693 patent) or with high-styrene (&gt;50 wt. %) resins (see U.S. Pat. No. 5,646,225), and not for aqueous polyurethane coatings.
A second common way to tweak the 2K aqueous polyurethane coating formulation is to modify the polyisocyanate. Most of the work to date has used a polyisocyanate modifed by partially reacting it with a hydrophilic polyether (see, e.g., U.S. Pat. Nos. 5,200,489, 5,194,487, 5,389,718, and 5,563,207). Making the polyisocyanate hydrophilic provides an emulsifiable crosslinker having improved compatibility with the co-reactants. This approach also has disadvantages, however. First, the hydrophilic polyisocyanate must be synthesized. Second, more of the expensive hydrophilic polyisocyanate must be used (compared with the unmodified polyisocyanates) to get the same NCO functionality contribution. Third, the hydrophilicity of the polyisocyanate is incorporated into the coating, often making its water sensitivity unacceptably high.
A third approach modifies the processing while keeping a commercial polyisocyanate in the formulation. The key concern is how adequately to disperse the polyisocyanate in water because emulsions made from commercial polyisocyanates tend to aggregate and settle. In one method, the particle size of the polyisocyanate is reduced by high-shear mixing (see the Jacobs article cited above). Unfortunately, high-shear mixing is energy-intensive, time-consuming, and requires special equipment. Adding cosolvents and emulsifiers can help, but this at least partially defeats the purpose of using an aqueous system.
Improved aqueous polyurethane coating compositions are needed. Preferably, the compositions would allow formulators to significantly reduce the levels of VOCs and HAPs present in paints and coatings. Preferably, the compositions are two-component systems without the physical property disadvantages of coatings based on aqueous PUDs. An ideal two-component system would use commercial polyisocyanates and yet would not require high-shear mixing. In addition, the ideal formulation would eliminate any need to make polyol components from expensive hydroxyalkyl acryate monomers. Finally, the industry would benefit from aqueous 2K polyurethane formulations that give coatings with an excellent balance of physical properties, including high gloss, hardness, impact resistance, flexibility, weatherability, and chemical resistance.